Carbon Monoxide (CO)

Test Overview

A carbon monoxide blood test is used to detect poisoning from breathing carbon monoxide (CO), a colorless, odorless, poisonous gas. The test measures the amount of hemoglobin that has bonded with carbon monoxide. This amount is also called the carboxyhemoglobin level. See a picture of carbon monoxide poisoning.

When a person inhales carbon monoxide, it combines with the red blood cells that normally carry oxygen to the body's tissues and replaces the oxygen that is normally carried in the blood. As a result, less oxygen is carried to the brain and other body tissues. Carbon monoxide can cause severe poisoning and death.

Carbon monoxide is made during burning when there is not enough oxygen present for complete combustion. The main sources of carbon monoxide are automobile exhaust fumes, fires burning with poor ventilation (such as gas heaters and indoor cooking fires), factories, and smoking tobacco.

Why It Is Done

A carbon monoxide blood test is used to detect poisoning from breathing carbon monoxide. You might have this test if you have been exposed to the gas or if you have unexplained symptoms, such as:

  • Headache, dizziness, or vision problems.
  • Nausea or vomiting.
  • Muscle weakness.
  • Confusion or trouble thinking.
  • Extreme sleepiness.

How To Prepare

Do not smoke before you have this test.

Talk to your doctor about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results may mean. To help you understand the importance of this test, fill out the medical test information form(What is a PDF document?) .

How It Is Done

The health professional drawing blood will:

  • Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
  • Clean the needle site with alcohol.
  • Put the needle into the vein. More than one needle stick may be needed.
  • Attach a tube to the needle to fill it with blood.
  • Remove the band from your arm when enough blood is collected.
  • Put a gauze pad or cotton ball over the needle site as the needle is removed.
  • Put pressure on the site and then put on a bandage.

How It Feels

The blood sample is taken from a vein in your arm. An elastic band is wrapped around your upper arm. It may feel tight. You may feel nothing at all from the needle, or you may feel a quick sting or pinch.

Risks

Blood test

There is very little chance of a problem from having blood sample taken from a vein.

  • You may get a small bruise at the site. You can lower the chance of bruising by keeping pressure on the site for several minutes.
  • In rare cases, the vein may become swollen after the blood sample is taken. This problem is called phlebitis. A warm compress can be used several times a day to treat this.
  • Ongoing bleeding can be a problem for people with bleeding disorders. Aspirin, warfarin (Coumadin), and other blood-thinning medicines can make bleeding more likely. If you have bleeding or clotting problems, or if you take blood-thinning medicine, tell your doctor before your blood sample is taken.

Results

A carbon monoxide blood test is used to detect poisoning from breathing carbon monoxide (CO), a colorless, odorless, poisonous gas. The test measures the amount of hemoglobin that has bonded with carbon monoxide. This amount is also called the carboxyhemoglobin level.

Carbon monoxide results are reported as a percentage: The amount of carbon monoxide bound to hemoglobin is divided by the total amount of hemoglobin (and then multiplied by 100). The higher the percentage, the greater the risk of having symptoms of carbon monoxide poisoning. With values below 10%, a person may not have any symptoms of poisoning.

Results are usually available right away.

Normal

Carbon monoxide
Nonsmokers:

Less than 3% of total hemoglobin

Smokers:

2%–10% of total hemoglobin

High values

High blood carbon monoxide values are caused by carbon monoxide poisoning. Symptoms of carbon monoxide poisoning become more severe as the carbon monoxide levels increase.

Symptoms related to high carbon monoxide values
Percent of total hemoglobin Symptoms

20%–30%

Headache, nausea, vomiting, and trouble making decisions

30%–40%

Dizziness, muscle weakness, vision problems, confusion, and increased heart rate and breathing rate

50%–60%

Loss of consciousness

Over 60%

Seizures, coma, death

Women and children may have more severe symptoms of carbon monoxide poisoning at lower carbon monoxide levels than men because women and children usually have fewer red blood cells.

What Affects the Test

Exposure to carbon monoxide from smoke, gas-burning equipment at home, heavy car traffic, or poorly ventilated car garages and factories can increase blood levels.

What To Think About

  • A person with symptoms and possible exposure to carbon monoxide, such as someone who lives in a house with an old heating system and complains of ongoing headaches, should be tested for carbon monoxide poisoning.
  • A person who may have carbon monoxide poisoning should be removed from the place of likely exposure and given oxygen to breathe before being tested.
  • The level of oxygen in the blood is usually within the normal range in people with carbon monoxide poisoning.
  • People who are regularly exposed to car exhaust, such as taxi drivers and traffic police, often have high carbon monoxide levels (8% to 12%).
  • If carbon monoxide poisoning is suspected, other tests (such as arterial blood gases and a complete blood count) may be done. An arterial blood gas (ABG) test may be done to determine whether symptoms are caused by carbon monoxide poisoning or by another disease that causes similar symptoms. See the medical tests Arterial Blood Gases (ABG) and Complete Blood Count (CBC).

References

Other Works Consulted

  • Chernecky CC, Berger BJ, eds. (2004). Laboratory Tests and Diagnostic Procedures, 4th ed. Philadelphia: Saunders.
  • Fischbach FT, Dunning MB III, eds. (2004). Manual of Laboratory and Diagnostic Tests, 7th ed. Philadelphia: Lippincott Williams and Wilkins.
  • Pagana KD, Pagana TJ (2006). Mosby’s Manual of Diagnostic and Laboratory Tests, 3rd ed. St. Louis: Mosby.

Credits

Author Maria G. Essig, MS, ELS
Editor Susan Van Houten, RN, BSN, MBA
Associate Editor Tracy Landauer
Primary Medical Reviewer Caroline S. Rhoads, MD - Internal Medicine
Specialist Medical Reviewer R. Steven Tharratt, MD, MPVM, FACP, FCCP - Pulmonology, Critical Care, Medical Toxicology
Last Updated March 18, 2008

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